Process for pulping bagasse with ammonium hydroxide and oxygen



United States Patent 3 274 049 PROCESS FOR PULP lN BAGASSE WITH AM- MGNIUM HYDROXIDE AND @XYGEN Marcel Mathew Gaschke, Toms River, and Marshall L.

Specter, Livingston, N..l., assignors to Pullman Incorporated, Chicago, llL, a corporation of Delaware N0 Drawing. Filed Feb. 25, 1965, Ser. No. 435,367

1 Claim. (Cl. 1162-65) This application is a continuation-in-part of our prior and copending application Serial No. 152,142, filed November 14, 1961, which in turn is a continuation-in-part of application Serial No. 124,425, filed July 17, 1961, both now abandoned.

This invention relates to an improved pulping process. In one aspect the invention relates to a new and improved pulping process particularly useful for pulping of nonwoody lignocellulosic material such as bagasse. In another aspect the invention relates to a pulping process in which the waste pulping liquor is such that it is utilizable as fertilizer.

Paper and paper products are manufactured from lignocellulosic material such as wood which has been converted into pulp. The pulp may be produced by any one of a number. of processes. Generally speaking, wood pulp is prepared by mechanical, chemical or semi-chemical processes. The primary advantage of the mechanical and semi-chemical methods is that the lignocellulosic material is processed either without chemical treatment or only under mild chemical conditions which merely soften the wood to facilitate subsequent mechanical separation of the pulp. The paper produced by such methods has low strength due to the high lignin residue and the short broken fibers in the ground pulp. Thus pulp produced by chemical methods is generally required for the manufacture of better quality paper, the most commonly employed commercial methods being the so-called sulfite process and the kraft process. Due to the large volume of pulp now being used in industry, presently available sources of raw materials such as woody lignocellulosic material are rapidly becoming inadequate in many areas of the world. It has become necessary, therefore, to find new sources of raw materials for the production of pulp. One such source is fibrous non-woody lignin-containing plant materials such as bagasse which is the fibrous residue remaining after the crushing and extraction of sugar cane, necessary steps in the production of sugar. The chemical composition of bagasse of various varieties is given in Pulp and Paper, volume 1 (Interscience Publishers, Inc., second edition, 1960) by James P. Casey at page 418. Although the holocellulose content of bagasse is comparable to that of hardwoods, bagasse contains considerably larger amounts of pentosans than woody lignocellulosic material. The chemical methods employed to produce bagasse pulp such as the neutral sulfite process have several drawbacks. One is that the paper produced therefrom is brittle due to the fact that the pulping medium does not effectuate substantial removal of the pentosans. Another is that, generally speaking, chemically pulped bagasse is unsuitable for the manufacture of newsprint unless the fibers are treated during the process with an acidic material, but even then manufacturers employing bagasse have consistently employed relatively large amounts of expensive fillers to render the paper product more suitable as newsprint. A chemical method for producing pulp of improved purity from bagasse and in high yield is highly desired by the pulp and paper industry, especially in sugar cane-producing areas and in areas where forest land is not abundant or is diminishing.

In addition to the above problems associated with finding improved chemical methods suitable for pulping of lignocellulosic material other than woody plants, problems arise in connection with disposal of waste liquors. It is known that cellulose, lignin, hemicellulose and vari- 3,274,049 Patented Sept. 20, 1966 ous other constituents such as resins, waxes, greases, pigments, tannin, etc., are found in plants in variable quantities according to whether they are resinous or leafy, hardwood or softwood type, or annual plants. Lignin, as well as some of the carbohydrate material, is solubilized and removed by chemical methods, the pulp waste liquor resulting therefrom containing a mixture of organic chemicals. The overall commercial value of the various chemical methods is handicapped due to the difiiculty of recovering even a portion of these organic materials. Although it has been common practice to dispose of the waste liquor by discharging it into streams, rivers or other bodies of water, objectionable water pollution caused thereby has become an ever increasing and serious problem. It is, therefore, highly desirable to provide a pulping process which results in a waste liquor that can be disposed of economically and advantageously and which does not cause pollution of inland streams.

It is an object of this invention, therefore, to provide an improved process for the manufacture of pulp from lignocellulosic material.

Another object is to provide a chemical pulping process particularly useful for the manufacture of pulp from nonwoody lignocellulosic material.

Another object is to provide a method for obtaining an inexpensive high-yield pulp from non-woody lignocellulosic material such as bagasse, which pulp is useful in the manufacture of printing paper, newsprint, containers, liner board, dissolving pulp and the like.

A further object is to provide a pulping process in which the nature of the pulping liquor is such that it is utilizable as a commercial product or disposed of without causing a pollution problem.

A further object is to provide a pulping process particularly useful for producing high purity cellulose in good yields from bagasse.

A further object is to provide a high yield chemical pulping process for producing readily bleachable pulp.

A still further object of this invention is to provide a simple and rapid process for the chemical pulping of bagasse and other agricultural residues whereby high purity pulp is produced economically and whereby the waste pulp liquor is advantageously disposed of by dumping on agricultural land.

Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.

Accordingly the above objects are accomplished by the process which comprises treating lignocellulosic material in an aqueous medium having a pH of at least about 9 and containing ammonium hydroxide as essentially the sole alkaline producing reagent, with oxygen or oxygencontaining gases under conditions such that cellulosic pulp is produced as a product of the process. In order to obtain a satisfactory pulping action and yield of pulp, the pH need be no higher than about 12.2. The pulping conditions include a temperature between about C. and about C. Within this temperature range the oxygen partial pressure ranges between about 5 and about 500 pounds per square inch (p.s.i.) and the residence time may vary from 2-3 minutes (at 170 C.) to about 10 hours (at 80 C.) The process of this invention is applicable to a variety of lignocellulosic material including hardwoods and softwoods as well as agricultural residues derived from non-woody lignocellulosic plants. A significant advantage of the pulping process of this invention is that the nature of the spent pulping liquor is such that it is utilizable as a plant nutrient.

The pulping process of this invention occurs without substantial oxidative degradation of the product, notwithstanding the particlar alkaline pH and the presence of oxygen. Although the process is applicable to the pulping of various species of wood such as spruce, birch, pine, poplar, beech, cedar, balsam and Douglas firs, it is described with particular reference to the pulping of agricultural residues such as bagasse, cornstalks, cotton stalks, soybean stalks, cereal straws and weeds as well as esparto, flax, hemp, jute and bamboo.

Generally speaking, a yield of total pulp of about 70 percent by weight of the lignocellulosic material employed as the raw material, indicates significant pulping whereas a yield of below about 40 percent by weight indicates that pulp which formed initially has been degraded. In addition to the desirability of obtaining total pulp yields within the aforesaid limits, it is desirable to produce pulp in which the holocellulose is composed principally of alpha-cellu lose with only minor amounts of carbohydrates such as pentosans and lignin, i.e., an amount sufficient to act as a binder of the fibrous cellulose and yet low enough so that paper manufactured from the pulp is not unduly brittle.

The process of this invention is particularly useful for the pulping of sugar cane bagasse. It is snow that the pentosan content of bagasse is quite high, and varies from about to about percent by weight depending upon the geographical source. It has been found that under the conditions employed herein, more than 70 percent of pentosans are removed. This result is surprising in view of the relatively short cooking times which may be employed in carrying out the process of this invent-ion. The aqueous ammonium hydroxide-oxygen system also selectively removes lignin without too much degradation of the alpha-cellulose and leads to a high yield of high purity bagasse pulp having good bleachability and properties useful in the manufacture of improved quality product paper. Notwithstanding selective removal of carbohydrate material such as pentosans, and of lignin, sufficient amounts remains to provide the necessary strength properties. Thus, yields of bagasse pulp of from about to about percent by weight, based on the weight of depithed raw material, are obtained by the process of this invention of which between about 70 and about per cent is alpha-cellulose. It is to be understood that, although yields of pulp derived from bagasse as expressed herein are based on the weight of depithed and dried bagasse, the bagasse raw material may comprise whole or depithed product, and need not be subjected to drying prior to use.

In carrying out the process of this invention, the lignocellulosic material is charged to the digestion zone where it is contacted with the aqueous ammonium hydroxide medium. Intimate contact is preferably maintained between the oxygen and aqueous medium and is accomplished by passing oxygen into the ammonium hydroxide solution at one or more points such that oxygen is dispersed throughout the solution. The partial pressure of oxygen may vary over a relatively wide range and is generally between about 5 and about 500 p.s.i. and is usually between about 25 and about 250 p.s.i. Although the oxygen partial pressure may be above 500 p.s.i., e.g., as high as about 800, it is unnecessary to operate at such elevated partial pressures since excellent yields of pulp are achieved within the lower ranges. It is to be understood that the term oxygen as used herein includes pure molecular oxygen as well as air and other oxygen-containing gases such as mixtures of oxygen and nitrogen in all mole ratios of O zN as well as other inert gases such as argon.

The aqueous ammonium hydroxide pulping liquor contains dissolved ammonia in an amount sufficient to maintain the pH of the medium value of at least about 9 for the entire or greater part of the cooking time. In effecting the presently described process, the starting materials are the lignocellulose-containing raw material, molecular oxygen or other oxygen-containing gas, water and ammonia. The digester is charged either with water to which ammonia is introduced, or a previously prepared solution of ammonium hydroxide. The lignocellulosic materials such as bagasse and oxygen are then introduced and the digestion effected at a temperature preferably between about 90 C. and about 160 C. and a digestion time from about 0.1 (at 160 C.) to about 4 hours (at 90 C.). As indicated, the higher temperatures are used in conjunction with the lower digestion times which are sufficiently low to render the process suitable for continuous operation. In accordance with the particularly preferred teaching of this invention, the pulping reaction is effected at a temperature between about C. and about C. for a digestion time of from 0.5 to 1.5 hours in an aqueous ammonium hydroxide solution having a pH from about 9 to about 12.2.

The amount of ammonia dissolved in the aqueous medium varies between about 0.01 and about 1.5, more usually between about 0.02 and about 0.5, pound mols of ammonia per pound of lignocellulosic material on a dry basis. The concentration of the aqueous ammonium hydroxide solution varies over a relatively wide range such as between about 0.1 and about 30 weight percent. More usually, however, the aqueous pulping liquor contains between about 0.5 and about 15 weight percent of dissolved ammonia.

The pulp product discharged from the digestion zone is ready for further processing by post-pulping procedures known to those skilled in the art such as separation from the pulping liquor, washing and bleaching, as desired. The chemical bagasse pulp, for example, is then used in the manufacture of boards, coarse papers, and highquality paper products. It also is advantageously employed for the manufacture of newsprint andfor this purpose it may be used in combination with bagasse pulp produced by mechanical or semi-mechanical methods. For example, satisfactory newsprint is manufactured by blending mechanically ground bagasse with bagasse pulp produced by the process described herein. In addition, newsprint may be produced by blending the chemical bagasse pulp produced by the process of this invention with mechanically ground wood pulp.

The pulping liquor withdrawn from the digestion zone is advantageously employed as a plant nutrient. Inasmuch as the pulping medium contains essentially no ions of metals of groups IA and HA of the periodic chart of the elements, nor sulfur-containing compounds such as those present in spent liquor from conventional processes, the spent or partially spent liquor is disposed of by discharging it onto agricultural lands. This may be accomplished by simple dumping of the liquor on the land or by pumping the liquor from the pulp-liquor separation zone through conduits connecting the pulping plant to an irrigation system or by means of conduits leading directly to the agricultural land. Waste pulp liquor contains, in addition to nitrogen in the form of dissolved ammonia, organic nitrogen compounds which form during the cooking period, each form serving as a valuable source of nitrogen or plant nutrient. It also is within the scope of this invention to pass the pulping liquor through a zone prior to being processed for use as fertilizer, in which zone free ammonia may be recovered for further use in the pulping process, or in which other valuable organic chemicals may be recovered. In accordance with this latter embodiment of the present invention, the residual liquor is still advantageously employed as fertilizer in view of the organic nitrogen-containing compounds. Alternatively, the pulping liquor separated from pulp product may be recycled to the digestion zone for further use therein with ultimate utilization of spent residual liquor as fertilizer. In accordance with another embodiment of this invention, the pulping liquor is treated with calcium hydroxide to release ammonia from ammonium salts, subjected to distillation to remove ammonia, and then discharged onto agricultural lands.

It is within the scope of this invention to include a pretreatment step by which the lignocellulosic material to be pulped is treated so as to facilitate diffusion of the pulping liquor into the pores of the material. Where bagasse is the raw material, it is generally unnecessary to include a pretreatment step, but in the case of pulping of woody lignocellulose, a pretreatment step may be beneficial. For example, the wood may be dried prior 5 to treatment with the pulping liquor. In addition, the air and moisture in the wood may be removed by placing the wood under vacuum, followed by introduction of the pulping liquor under pressure. Another method is a compression-type impregnation with the use of a laboratory Sturtevant roller mill. With this system the chips are initially steeped in treating solution, then subjected to repeated passes through the mill, allowing the pressed chips to drop into a pool of liquor. Another pretreatment method is by the use of a continuous roller mill such as the Haug-Kollermill. Steeped chips enter at one end and are thrown into the solution by centrifugal force. As the cylinder rotates, they are caught in the nip between the rolls and cylinder, both of which are driven. Then as the chips leave the nip, they enter a fog of treating chemical.

The following examples are offered as a better understanding of the present invention and are not to be construed in a limiting sense.

The following experiments were carried out in a stainless steel assembly consisting of a digester made of an externally heated pipe (2 feet in length and 4 inches in diameter). The pipe was provided with a welded cap, a gas inlet consisting of a porous metal disc, and a sampling connection on the lower end and a slip-on flange on the other end. The digester was connected to a condenser followed by a back pressure regulator and rotameter. At the start of each run, the raw material to be pulped was charged to the digester and sufiicient .aqueous pulping liquor was added to fully immerse the feed. The digester was then closed and oxygen introduced through the porous metal disc immersed in the pulping liquor. The oxygen was introduced at a measured rate, and the oxygen was passed through the reaction mixture, condenser, back pressure regulator, valve and rotameter. The pressure drop occurred after the valve and the flow rates were measured at atmospheric pressure. The flow rates set forth below have been calculated to standard conditions of pressure and temperature. Since the system did not contain an internal agitator, mixing was provided entirely by passage of gas through the liquor. The calculated partial pressure of oxygen was measured as the difference between the total pressure and the partial pressure of aqueous ammonia at temperature. At the end of each run, the hot pulp liquor was discharged through an outlet at the bottom of the digester and the system was thus depressurized. A liner containing the pulp product was removed from the reactor and the pulp washed on .a screen, and the yield determined on the basis of oven dried pulp to raw material on a dry basis. The pH values were measured with a Beckman Model G pH meter using glass and calomel electrodes. The values are considered accurate within 10.2 pH unit.

EXAMPLES ll3 A series of experiments was conducted, using depithed bagasse containing, on a dry basis, 22.8 percent lignin, and 70.4 percent holocellulose of which approximately 26 percent is pentosan. In each experiment the depithed bagasse grams on a dry basis) was charged to the above-described digester and immersed in 2 liters of aque ous ammonium hydroxide solution having a pH of approximately 11.5. In each experiment the oxygen partial pressure and oxygen flow rate were maintained constant at 100 p.s.i. and 7 standard liters per minute, respectively. The amount of ammonia in the aqueous solution, temperature and cooking time employed in each run are tabulated in the following Table I which also sets forth the total yield, lignin and holocellulose contents and the permanganate number of the pulp product produced total pressure was ad usted to the desired value. Gaseous in each run.

Table 1 Example 1 2 3 4 5 6 Pulp Source Bagasse, 100 grams Oxygen Feed:

Partial Pressure, p.s.i 100 100 100 100 100 100 Flow Rate, standard 7 7 7 7 7 7 Total Pressure, p.s.i.g 165 193 180 165 193 182 Aqueous Ammonium Hydroxide: NH

milliequivalents/gram bagasse 29. 4 88. 2 58.8 29.4 88. 2 29.4 Temperature, C 130 130 130 130 Time, hours 0. 5 0.5 1 1. 5 1. 5 1 Total Pulp Yield, weight percent 1 60. 7 1 58. 6 54. 6 60. 1 56. 2 56. 6 Percent Lignin 14. 5 14.7 8.1 9. 7 10. 2 10.3 6. 4 11.0 Percent Holocellulose 82. 1 81. 6 89. 5 86. 9 87. 9 87. 1 93. 7 87. 3 Percent Lig'nin (basis, bagasse) 1 8. 9 1 5. 2 5. 6 6. 2 3. 6 6. 2 Percent Holocellulose (basis, bagasse) 1 49. 7 1 51. 7 48.0 52. 4 52. 7 49. 4 Permanganate number. 16. 3 18.2 14. 3 19.9 19. 9 17.3 15.8 20. 4

Example 7 s 9 10 11 12 13 Pulp Source Bagasse, 100 grams Oxygen Feed:

Partial Pressure, p.s.i 100 100 100 100 100 100 100 Flow Rate, standard liters/ minute 7 7 7 7 7 7 7 Total Pressure, p.s.i.g 197 210 197 202 238 202 238 Aqueous Ammonium Hydroxide:

N11 milliequivalents/grarn bagasse. 58. 8 88. 2 58. 8 29. 4 88. 2 29. 4 88. 2 Temperature, C 140 140 140 150 150 150 Time, hours 1 1 1. 5 0.5 0.5 1. 5 1. 5 Total Pulp Yield, weight percent 59. 9 56. 1 54. 6 59.6 60.6 48. 3 52. 9 Percent Lig'nin 8. 7 7. 2 9. 2 9. 7 9. 5 7. 8 8. 5 Percent Holocellulose 88. 8 90. 6 89. 6 91. 2 89. 2 93. 0 89. 9 Percent Lignin (basis, bagasse). 5.3 4 5.0 5. 8 5.8 3. 8 4. 5 Percent Holocellulose (basis,

bagasse) 53. 2 50. 8 48. 9 54. 4 54. 1 44. 9 47. 6 Permanganate number. 19. 1 16.8 19. 6 l9. 0 19. 0 13. 7 19. 1

1 Average of 2 duplicate runs conducted under the indicated conditions. 2 Average of 6 duplicate runs conducted under the indicated conditions.

Inspection of the results set forth in the above table shows that the process of this invention leads to excellent yields of bagasse pulp of good bleachability, notwithstanding the relatively mild operating conditions of temperature and alkaline pH, and short reaction times. Although the pH of the pulping liquor gradually decreases during the pulping reaction, substantially all of the pulp product is formed at pH values above 9. For example, the pH values of the pulping liquor as measured at the end of the digestion times of Examples 1, 5, 7 and 11 Were 9.98, 10.02, 10.21 and 9.78, respectively. Another important factor which renders the oxygen-aqueous ammonium hydroxide pulping medium a particularly valuable medium for the pulping of bagasse is that it effectuates substantial removal of pentosans. For example, upon analysis the pulp product from Example 12 was found to contain 16.7 weight percent pentosans, or 8.1 percent on the basis of the bagasse starting material, demonstrating that about 70 percent of the pentosan content of the depithed bagasse raw material was removed.

The following example illustrates the pulping of wood in accordance with the teachings of this invention.

EXAMPLE 14 The above-described reactor was charged with 100 grams of dried spruce flakes and 3.2 liters of 3.4 normal ammonium hydroxide. Oxygen was passed through the reaction mixture in the above-described manner, the partial pressure of oxygen being 246 p.s.i. at a total pressure of 300 p.s.i.g. Prior to the introduction of oxygen, the reaction mixture was allowed to stand at 25 C. for 70 minutes during which time the pH of the mixture was 12.2. The reaction temperature was then raised to 150 C. and maintained at this tem- .perature for 3 hours, during which time oxygen was introduced into the reaction mixture at the indicated pressure. The pH of the reaction mixture was measured at one half hour intervals, the values being as follows.

Reaction time at 150 0., hours: pH 11.02

At the end of the 3hour reaction period, the product was separated, the yield of pulp being 49.4 Weight percent.

Similar pulping reactions are eifected under the conditions of the above examples using other lignocellulosic materials of the non-woody and woody types to produce good yields of fibrous pulp. It is to be understood that the process of this invention is also applicable to the pulping of mixtures of the various lignocellulosic materials described herein.

It is apparent that by the process of this invention an improved method is provided for pulping of lignocellulosic material whereby a high yield of good quality pulp is produced and substantially complete utilization of chemicals is realized. The process is especially attractive in sugar cane-producing areas where an economical and satisfactory chemical process for pulping of bagasse has long been desired, and is also attractive in arid areas where it is mandatory that efiluent liquor be innocuous.

Various modifications and alterations of the process of this invention Will become apparent to those skilled in the art from the accompanying description and dis closure without departing from the scope of this invention.

Having described our invention, we claim:

A process for pulping of bagasse to produce fibrous pulp which comprises treating bagasse in an aqueous alkaline solution containing ammonium hydroxide as essentially the sole alkaline producing reagent and having a pH from about 9 to about 12.2, dispersing a free oxygen-containing gas through said aqueous alkaline solution, the partial pressure of the oxygen being between about 25 and about 250 pounds per square inch, maintaining the aqueous reaction medium at a temperature between about C. and about C. for a digestion time between about 0.5 and about 1.5 hours, to produce fibrous pulp in a yield not less than 40 Weight percent expressed on the basis of dried and depithed bagasse.

References Cited by the Examiner UNITED STATES PATENTS 1,780,842 11/1930 Richter 16229 1,802,715 4/ 1931 Hixon 16-290 1,887,899 11/1932 Bradley 1625 3 2,243,050 5/1941 Plant l6265 2,686,120 8/1954 Marshall 16265 DONALL H. SYLVESTER, Primary Examiner.

H. CAINE, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 274, 049 September 20, 1966 Marcel Mathew Gaschke et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 67, after "medium" insert at a column 4, line 68, before "pulping" insert spent Signed and sealed this 22nd day of August 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Offioer Commissioner of Patents 

